The sport has evolved through rule changes, training, coaching, higher participation in the sport and PEDs.
Fighters are much more well rounded, due to the advancement of training, coaching, PEDs and the increased number of participants in the sport which leads to a high level of competition. That's why WMMA is still pretty bad because there isn't enough participants to create defined separations in skill level (like champions, elite, gate keeper, prelim fighter, regional fighter, can, taxi driver, etc). Regional WMMA fighters fight for world titles in big promotions. That doesn't happen in the mens divisions. Obviously some fighters aren't well rounded and rely on their specialities but the top guys try to be elite at everything.
All the GOATS were elite fighters because of how well rounded they were and they would be elite in any era. Mindset and experience are pretty big aspects of being elite as well.
Fighters can only evolve if humans evolve. Which, I haven't seen any examples of human evolution in the last 30 years.
tldr; there are more elite fighters in MMA because there are more fighters in general.